Rigid riser hang-off devices are important pieces of equipment in offshore engineering. Until now two types of devices have been used for that purpose:                Flex joints according to U.S. Pat. No. 5,269,629 by Langner,        Tapered Transition Joints (TSJs).Those are primarily used for hang-offs or Steel Catenary Risers (SCRs), which are the most common, including steel lazy wave risers. Other relevant types of rigid risers are for example Chinese lantern riser, bottom weighed riser, etc., and rigid catenary jumpers ‘U’ and ‘W’ shaped.        
The use of the flex joints is limited to low and medium design temperatures and low to medium design pressures. At high temperatures and high pressures (HTHP) the only devices that have been used to date are TSJs. The use of steel TSJs to handle angular deflections at high tensions is limited to small deflection angles. At high effective tension and large angular deflections a steel TSJ would have been prohibitively long, and that is why titanium TSJs are used, see OTC 18624.
The load characteristics, design details and limitations of both flex joints and TSJs are well known to those skilled in the art.
Titanium TSJs and flex joints are mostly used in standard SCR top deflection angle ranges with the upper limit of approximately 20° to 25°. For greater deflection angles titanium TSJs would have been very long and expensive. Two stage flex joints can handle deflection angles up to 35° to 40°, albeit at considerably increased costs. Flexible risers are therefore used in medium and shallow water applications that require high deflection angles; however, flexible pipe cannot handle HTHPs.
In the nineties deployment of SCRs supported from a turret of a Floating Production Storage and Offloading (FPSO) vessel for Gulf of Mexico (GoM) project(s) were considered. Shell investigated a use of flex joints utilizing typical receptacle baskets, which was proven feasible but all details are confidential. The first turret moored FPSO utilizing lazy wave SCRs BC-10 (Shell) was installed in 2009 offshore Brazil, inclined I-tube pulled SCRs utilizing flex joints were used. The first disconnectable turret FPSO using SCRs (Shell Stones) should start production in the GoM in 2016. WO2014180667 by Cao et al also features inclined I-tubes for steel risers used with a disconnectable turret buoy.
The third type of hang-off devices is the so called ‘spiralflex’ disclosed in U.S. Pat. Nos. 8,550,171 and 8,689,882, Australian Patents 2008,211,995 and 2010,238,542 and in European Patent 2,042,682 by Wajnikonis and Leverette. The spiralflex separates the structural functions:                The riser is suspended from a pivot that transfers riser tension to the vessel;        Fluid transport, pressure containment and angular deflections are accommodated by spiralflex spools that in all the implementations disclosed by U.S. Pat. No. 8,550,171 etc. consist of entry spools, coils (spirals) and exit spools.        
Wajnikonis and Leverette claim the use of catenary riser hang-off clamps with fluid conduit coils featuring two or more turns. There are no known offshore applications of the spiralflex to date.
Spiralflex hang-off according to Wajnikonis and Leverette is difficult to install. There are difficulties to work around a need to thread handling equipment like shackles, crane hooks, tri-plates, etc. through the coils. Another problem is that most designs by Wajnikonis and Leverette would require indirect two or more lifting points of the SCR by using pulleys, additional handling equipment, etc. that would be difficult to control. Another group of problems concerns the assembly, handling and protecting the coils during S-lays, J-lays or reel-lays, in cases of riser installations with Spiralflex spools assembled on risers. Spiralflex has problems in handling high deflection angles—high pitch implying large dimensions may be required to prevent closing up gaps between neighboring coil turns. Spiralflex designs tend to have large size and would be difficult or impossible to adapt for coiled tubing or wireline interventions. Because of the size of the coil, the riser porch may need to be offset more from the vessel side, which increases static bending moments acting on the porch and on the vessel structure adjacent. These static loads would be higher than those pertaining to the use of flex joints or titanium TSJs.
Spiralflex uses a fairly long three-dimensional (3-D) pipe with all the natural oscillation modes of a beam of the same length, plus additional low frequency modes. The latter are present because of the large effective lump mass of two or more turns coils that are suspended on the entry and exit spools forming slender, compliant member-springs. Another group of (predominantly) low frequency modes are governed by axial oscillation modes of coils. Those features imply greater vortex induced vibration (VIV) susceptibility of spiralflex spools than is that of straight pipe spans or of rigid jumpers of the same lengths.
Another issue is flow assurance. SCR tend to be designed with an angular offset from the vertical at the hang-off locations. Those offsets are typically of the order of 10°. The hang-off angles are usually greater than the slope angles in spiralflex coils are, which imply alternating ascending and descending flow when the product flows around the spiralflex coil. To alleviate that the coil pitch may be increased which would imply increases in the coil length. The alternating ascending and descending coil segments would allow stagnant liquids to remain in the lower parts of coil turns promoting hydrate formation and corrosion.
For SCR installations featuring flex joints and TSJs the top of the riser is one of the two riser regions that are loaded the highest in fatigue, the other one is the touch-down zone (TDZ). On fields where the seabed is soft like in the Gulf of Mexico, much of Offshore Brazil and Offshore West Africa, the TDZ fatigue damage may be not as high as is that near the riser top, see OTC 15104. Bending fatigue is the main potential failure mode for risers. At the riser top it is governed by the rotational stiffness of the hang-off and the bending fatigue damage can be very high for rigid risers utilizing TSJs and still fairly high for flex joint hang-offs. Riser fatigue is often the main criterion for the selection of a type of a floater to be used. Whenever the riser fatigue is limited, in particular the bending fatigue, less expensive vessels can be used. Those would be semi submersibles or ship-shaped FPSOs or Floating Storage Offloading (FSOs) vessels that move more on waves. Otherwise moving less, but more expensive vessels like Tension Leg Platforms (TLPs) or Spar platforms must be used.